The present invention relates to solid electrolytic cells of monolithic structure and more particularly to sintering methods and sintering aids for forming such monolithic structures.
Fuel cells and other electrolytic cells that do not include liquid electrolyte have been the subject of considerable study. Representative of this technology are U.S. Pat. Nos. 4,476,198 to Ackerman et al.; 4,476,197 to Herceg; 4,476,196 to Poeppel et al.; and 4,510,212 to Fraioli, wherein each relate to certain aspects of solid oxide fuel cells or electrolytic production cells.
As is described in several of the above cited patents, monolithic electrolytic cells include two general wall types defining passageways for reactant gases. One wall includes a dense layer of (Zr, Y)O.sub.2 electrolyte interposed between two porous electrode layers. This electrolyte wall permits only O.sup.2- ions to flow when a gradient in oxygen pressure is applied, generating the desired electrical current. The second wall type includes an electronically conductive, interconnect material between electrode layers of opposite polarity, each electrode layer being of a distinct but adjacent cell. It is to this second type wall that the present invention is addressed in particular.
Lanthanum chromite (LaCrO.sub.3) is a refractory of considerable interest for use in interconnect layers or monolithic fuel cell stacks. This electrically conductive refractory may also have a value as a heating element in a high temperature furnace.
Formidable problems have arisen in fabricating a monolithic structure with many thin layers of different ceramic materials. The thin, frangible layers are not easily formed and assembled into an operable structure. An assembly of green layers requires similar firing characteristics for the various ceramic materials employed. This has been of particular consequence in respect to the interconnection material, lanthanum chromite. This material will densify only at temperatures above 1900.degree. K. under reducing conditions making it incompatable with fabrication techniques suitable for prospective electrode and electrolyte materials.
SUMMARY OF THE INVENTION
Therefore, in view of the above, it is an object of the present invention to provide an improved method for forming lamina including an electrically conductive lanthanum chromite layer.
It is also an object to provide sintering aids for lanthanum chromite to permit densification at reduced temperatures.
It is also an object of the invention to provide a method of forming a laminated interconnect wall for densification at a single sintering temperature.
It is a further object of the invention to provide lamina of green ceramic material suitable for densification by sintering.
In accordance with the present invention, a method is provided for forming an electronically conductive, interconnection layer including lanthanum chromite. The method comprises depositing a layer of particulate lanthanum chromite containing a minor fraction of a sintering aid having a melting point substantially below the melting point of lanthanum chromite. The layer is sintered at a sufficiently high temperture of not more than 1800.degree. K. to effect densification to more than 90%, preferably at least 94% theoretical density.
In the more specific aspects of the invention, the sintering aid includes an oxide of boron.
In another aspect of the invention, the sintering aid includes a eutectic affording composition of at least two components which composition provides a minor liquid phase in the layer at a temperature below 1600.degree. K.
In a further aspect of the invention, the eutectic affording composition is selected from the class of compositions including Group 2A-metal fluorides with Group 3B-metal fluorides, and Group 2A-metal oxides with Group 6B-metal oxides.
In another more specific aspect of the invention, the eutectic affording composition canb e selected from YF.sub.3 CaF.sub.2, YF.sub.3 -MgF.sub.2, LaF.sub.3 -CaF.sub.2, LaF.sub.3 -MgF.sub.2 and CaO-Cr.sub.2 O.sub.3.
In one other specific aspect, the sintering aid includes the combination of an oxide of boron with a eutectic affording composition, which composition provides a minor liquid phase at a temperature below 1600.degree. K.
In one other specific aspect, the oxide of boron is selected from the lanthanum borates, yttrium borates, boric acid or boron oxide.
The invention also comprehends a method of forming an electrolytic composite suitable for use as an interconnection wall between series connected electrolytic cells. The method includes providing a layer of first electrode material including an electrochemical catalyst, depositing on the first layer a layer of particulate lanthanum chromite with a sintering aid including an oxide of boron and a eutectic affording composition selected from the class of compositions consisting of metal compounds capable of providing a liquid phase at temperatures below 1600.degree. K., the metal compounds are selected from groups 2A, 3B and 6B metals combined with anions selected from the oxides and fluorides. A layer of second electrode material is deposited on the lanthanum chromite layer to form a three-layered structure, which structure is sintered at a temperature of 1400.degree.-1800.degree. K. to bond the layers into an integral lamination having an electronically conductive lanthanum chromite layer interconnecting the layers of first and second electrode materials.
In other aspects of the invention, each of the green layers are deposited by tape-casting techniques from a slip of particulate material, solvent, and polymeric binder selected to be driven off as vapor in the sintering step.
The present invention also contemplates a green laminated structure suitable for sintering at a temperature below 1800.degree. K. to form an integral three-layered wall for use as an interconnection between series connected electrolytic cells. The lamination includes a first layer of lanthanum-manganite with strontium doping as cathode material, a second layer containing lanthanum chromite and a sintering aid, the sintering aid includes an oxide of boron combined with a eutectic-affording composition selected from the fluorides and the oxides of the group 2A, group 3B and group 6B metals. The third layer in the lamination contains a cermet of a transition metal and stabilized zirconia for use as anode material.